The purpose of this project is to define the mechanism by which inorganic phosphate (Pi) crosses the intestinal cells. This will be accomplished by measuring Pi transport into and across the intestinal cells and to define the influence of various factors, e.g., 1,25-dihydroxyvitamin D3 (1,25D), on such processes. The goals set for the current year will be to further pursue the ongoing studies on the action and interaction of colon-active steroid hormones, 1,25D, glucocorticoids (GC) and mineralocorticoids (MC), on colonic Ca and Pi transport. Transport studies will mainly consist of the measurement of brush border influx, Jmc, and unidirectional transmural fluxes, Jms and Jsm, using isotopic tracers and in the absence of electrochemical gradients. Fluxes through the paracellular space will also be monitored using 3H-PEG-900 as a marker. The action of 1,25D, GC and MC on colonic Ca and Pi transport will be characterized by the measurement of Vmax and Km for brush border influx and transmural fluxes in both the proximal and distal colon. The action of these steroid hormones will be compared and contrasted with those resulting from in vitro modification of brush border permeability to Ca and/or Pi transport and each of the known components of Na transport in colon will be studied using substrate replacment technique and the application of both in vivo and in vitro metabolic inhibitors.